"fish-eating marine mammal with flippers; any pinniped not a walrus," Middle English sele, from Old English seolh "seal," from Proto-Germanic *selkhaz (compare Old Norse selr, Swedish sjöl, Danish sæl, Middle Low German sel, Middle Dutch seel, Old High German selah), a word of unknown origin, perhaps a borrowing from Finnic.
Seal point "dark brown marking on a Siamese cat" is recorded from 1934, from the resemblance to the color of seal fur; compare seal brown "rich, dark brown color," which is attested by 1875. Old English seolhbæð, literally "seal's bath," was an Anglo-Saxon kenning for "the sea."